Bath solution and a process of treating metal surfaces



2,809,138 Patented Oct. 8, 1957 ice BATH SOLUTION AND A PROCESS OFTREATING METAL SURFACES Ernst Wagner and August Moeller, Frankfurt amMain, Germany, assignors to Farbwerke Hoechst Aktiengeselb schaftvorrnals Meister Lucius & Briining, Frankfurt am Main, Germany, acorporation of Germany No Drawing. Application March 11, 1955, SerialNo. 493,834

Claims priority, application Germany March 18, 1954 Claims. (Cl.148--6.17)

The present invention relates to a bath solution and a process oftreating metal surfaces.

It is known to be generally impossible to cover highalloy steels, forinstance refined steels, by means of phosphatizing solutions with alayer of crystals intimately grown together. It has, therefore, alreadybeen proposed to use solutions of oxalic acid with addition of soluble,inorganic sulfides in order to obtain a well formed crystal layerfacilitating the cold shaping of refined steels. Solutions of alteredcomposition containing halogen ions, particularly chlorine ions, ironions and sulfurous acid or oxalic acid have also been proposed already.Furthermore, in order to produce oxalate layers on refined steelcontaining 18% of chromium and 8% of nickel, solutions have been usedwhich contain oxalic acid, ferric oxalate and bromide, ferric cyanide orthio-cyanide, the simultaneous use of sulfides, sulfites or thiosulfatesalso having been taken into consideration. Another known solution forpreparing oxalate layers substantially consists of an oxygen containingsulfur compound developing sulfur dioxide under the conditions of thebath and a soluble alkali metal halide as activator. As oxygencontaining sulfur compounds there are used, in this case, thiosulfate,hydrosulfite, tetrathionate or sulfite. When phosphate is added to theacid oxalate baths, the produced layers consist of oxalate and phosphatein varying composition.

It has proved a disadvantage of the known baths that the decompositionof the inorganic sulfur compounds takes place very rapidly, volatilecompounds being split off, the partially contaminating effect of which,together with the annoyance caused by the disagreeable odor arising overthe baths during the formation of the layers are extremely troublesomeand require precautionary measures.

The common characteristic of all these substances is the production of alayer by means of inorganic sulfur compounds to which iron salts andhalides are added in order to activate the surface of the refined steel.

Now, we have found a bath liquor for treating metal surfaces in order toproduce surface layers, especially on low-alloy and higher alloy steelsas well as on chromium-nickel alloys. When using this liquor, thedisadvantages connected with the use of the known liquors are avoided.The bath liquor according to the invention has an acid degreecorresponding to a pH range between about 1.0 and about 2.4 andcontains, in aqueous solution, oxalic acid anions as well as organicsulfur compounds soluble in the bath liquor and characterized by thegroups =C=S (thiourea (NH2)2=C=S); ECSH (thioglycolic acid HOOCH2C SH);i

of the oxalic acid can be produced in the bath liquor by.

adding oxalic acid or by using oxalates soluble in the acid bath liquor,especially alkali metal oxalates and ferric oxalates. Mixtures ofvarious oxalates or mixtures of an oxalate with oxalic acid may likewisebe used. The organic sulfur compounds characterized above may suitablybe added to the solution in the form of the alkali metal compounds ofsuch organic sulfur compounds. In particular, it is possible to operatewith bath solutions which contain, either per se or in every desiredmixture, the xanthates for instance methyl-, ethyl-, propyl-, butyl-,amylor hexyl-xanthate. Furthermore, solutions according to theinventioncan advantageously be used containing as organic sulfurcompounds dithiophosphoric acid esters of the aliphatic or aromaticseries. As a further example, solutions are mentioned which containthiourea, eventually in mixture with other organic thio compounds of theaforementioned groups. Bath solutions containing thioglycolic acid canalso be used.

The pH-value of the bath solutions can be adjusted in accordance withthe amount of oxalic acid or in accordance with the added quantity ofother acids, for instance also a corresponding quantity of sulfuricacid.

The bath solutions proposed according to the invention may, in addition,also contain P04 ions. There can, furthermore, also be present ions ofheavy metals, for instance those of the trivalent iron and of manganese.The P04 ions can be produced by adding phosphoric acid and/orphosphates, especially phosphates of heavy metals. The bath solutionsmay, in addition, contain accelerators, for instance sodium sulfite,sodium bisulfite, alkali metal thiosulfate, alkali metal hydrosulfite,alkali metal thiocyanate.

The amount of oxalic acid and oxalates contained in the bath solutionmay vary within wide limits. They can, for instance, be present in suchan amount that the oxalic acid anions contained in the bath solutionamount to a quantity between about 0.5 gram and about grams per liter ofbath solution. The quantity of organic sulfides proposed according tothe invention may vary between about 0.025 gram and about 5 grams perliter.

When the solutions according to the invention are used for the treatmentof metallic surfaces in order to produce surface layers, the operationat elevated temperature has proved advantageous, as it accelerates thedevelopment of the layer. Temperatures which have proved suitable rangebetween about 20 C. and about 98 C. When proceeding in this way, thebath liquor proposed according to the invention and containing, inaqueous solution, oxalic acid anions and at least one of theafore-mentioned organic sulfur compounds and which, furthermore, has anacidity corresponding to a pH value between about 1.0 and about 2.4, isheated to a temperature within the afore-mentioned range, and thesurface to be treated is contacted with this bath liquor by dipping itinto the latter. If need be, it is also possible to vary the termperature during the treatment.

The temperature depends, to a large extent, upon the composition of themetal surfaces to be treated. For instance, low-alloy steels can betreated at temperatures of about 20 C. to about 60 C. and high-alloysteels at temperatures between about 60 C. and about 98 C. Of course,all other eventual additions proposed according to the invention can beadded to the above mentioned bath solution which is cited as an exampleonly, viz. phosphoric acid, phosphates, manganeseand ironions. Theduration of the treatment when operating with the bath liquors proposedaccording to the invention is dependent, on the one hand, upon the shapeand the composition of the metal surface to be covered with a layer,and, on the other hand, also upon the temperature of operation. Theoperation times generally coming into consideration vary between about 2and about 40 minutes.

The bath solution is, for instance, prepared as follows: In a bath of100 liters, 6 kilos of oxalic acid are dissolved, 3.1 liters of asolution are then added which contains 32.65% of primary manganesephosphate and 4.5% of phosphoric acid.

Finally, 0.2 kilo of a mixture consisting of 0.02 kilo of thio-urea and0.18 kilo of sodium thiosulfate are added and the whole is well mixed.After heating the bath to 60 C.- 80 C. the refined steel par-ts are hungin and taken out of the bath when the reaction is finished, i. e. whenno more gas is evolved. After rinsing, they are dried in a currentof-hot air. If need be, they are dipped, before drying, into a neutralsoap solution, then they are dried and submitted to cold shaping. The pHvalue of this bath solution is equal to l.

Solutions containing oxalic acid, oxalates, phosphoric acid andphosphates of heavy metals'as well as organic sulfur compounds asproposed according to the invention in quantities of about 0.025 gram upto grams perliter, are of particularly good use. These solutions arepreferably suitable for the development of adhesive, tight coatings onrefined steels. These coatings favour good deforming action. Such a bathsolution, for instance, contains the following substances:

Crystallized oxalic acid in a quantity varying-between 1 gram to 200grams per liter, preferably 40 grams to 60 grams, phosphoric acid orphosphates in aquantity varying between -2 grams and 35 grams per literand calculated as P205, preferably 6 grams to 8.5 grams per liter, ifdesired iron in a quantity of 0.5 gram to '15 grams per liter, ifdesired manganese in a quantity of 0.05 gram to 3 grams per liter andorganic sulfides as proposed by the invention in an amount between about0.025 g. and about 5 grams per liter. The phosphates canbe added in theform of primary, secondary or tertiary alkali metal phosphates and'may,in this form, serve for'adjusting the pH value.

After the treatment, the coatings can be dried in the usual manner andthen'also' be submitted to the cold tion are preferably used for thetreatment of surfaces of iron, of alloy steels and unalloyed steels.However, they alsov allow the treatment of other metals or alloys, for

instance copper, zinc, aluminum, nickel and chromium. The metal surfacescan be in the form of sheets, Wires or in other shapes. 7

The. advantages ofrthe' bath solutions according to the inventionespecially consist in the factthat the organic sulfur compounds insolutions do not split off sulfur. Contrary to. inorganic sulfurcompounds, their decomposition, and. therewith their use in the bath,takes place extraordinarily slowly, for instance when thiophosphates areused, and in many cases, forinstancein thecase of thiourea, it is notascertainableat all. This means a much, reduced consumption of thesesubstances in the bath.

Another advantage is'represented bythe fact that, when using some of theorganic sulfur compounds proposedac cQrding-tothe invention, forinstance 'thiourea, the annoyar ee causedby the disagreeable-odor abovethe-baths is. considerably reduced or even eliminated. According.-

to their chemical behavior it could not be expectedthat: theorganicsulfur compounds would develop the; same or, in several, cases, forinstance, especially when thio;

phosphate is present, even a better efliciency than the inorganicsulfides. The esters of the thiophosphoric acid, which are suitablyadded to the baths in the form of alkali metal salts, as well asthiourea, remain as anions undecomposed on addition of an acid. Xanthicacid set free in acid solutions of alkali metal xanthates decomposesonly after a certain time into the components, i. e. into carbondisulfide and alcohol. The decomposition takes place the more slowly,the longer the carbon chain is.

In the following examples sheets of refined steel of varying compositionare treated with the bath solutions according to the invention.Previously, the sheets are freed from forge scales by pickling them inthe usual acid pickling liquors. This preliminary treatment can also beeifected by means of alkaline pickling liquors or melts. According toExamples 1-12 the sheets of refined steel are dipped into a standardsolution to which the substances mentioned for each case, according tothe invention, are also added; in Examples 1353 variations in thecomposition of the standard solution are described which are within theabove mentioned limits of the mixture. In each of the following Examples1-12 the pH value of the bath solution amounts to about 1.

Example 1 A sheet of refined steel containing less than .15 of 0,03% ofSi, 18.0% of Cr, 8.0% of Ni is dipped, for 10 minutes, into a solutionof C.- C. This solution contains per liter 60 grams of crystallizedoxalic acid, 3 grams of Mn, 9 grams of P205 and 0.25 gram of'potassiummethylxanthate. A dark, crystallized layer is obtained. If instead ofpotassium methyl xanthate, the same quantity of potassium ethyl xanthateis used, while all other conditions remain the same, there is alsoobtained a dark, crystallized layer. When using solutions which, as anorganic sulfide, contain per liter 0.25 gram of'potassiumisopropylxanthate, dark gray crystallized layers are obtained. Solutionscontaining the same quantity of potassium butyl xanthate result underthe same conditions in black-gray crystallized layers, whereas the useof solutions containing an equal quantity of potassium isobutyl xanthateas well as the use of an equal quantity of potassium amyl xanthateresult in gray, crystallized layers. Light gray crystallized layers areobtained when. a solution according to the above characteristics is usedwhich, instead of potassium methyl xanthate contains per liter 0.25 gramof potassium hexylxanthate and when the abovev mentioned operatingconditions are maintained.

Example 2 A sheet of'refined steel containing less than 0.1% of C, 0.3%of Si and 18% of Cr is treated, at a temperature of 85 C., with asolution containing per liter 60 grams of crystallized oxalic acid, 3grams of Mn, 9 grams of P205 as well as 0.25 gram of potassiumhexylxanthate. The treatment is finished after 2 minutes. The sheetshows awell formed, uniform layer. When maintaining the same conditions,but working at a temperature of 60 C., the. treatment takes about 7'minutes.

Example 3 Example 4 A sheet of refined steel containing 18% of Cr and 8%of Ni is treated for 10-15 minutes at a temperature of 9,0. to, 95." C;withv a. solution containing'per' liter 60 gramsof'crystallized'oxalicacid, 3. grams-of Mn, 9 grams acce ed of P205 as well as 2 grams each ofthe following esters of the thiophosphoric acid: (a) Potassium salt ofthe diethyl ester, (b) Sodium salt of the dibutyl ester, (c) Potassiumsalt of the p-dicresyl ester, (d) Sodium salt of the diphenyl ester.

Well crystallized layers are obtained.

Example 5 A sheet of refined steel containing 18% of Cr is treated for 4minutes at a temperature of 80 C. with a solution containing per liter60 grams of crystallized oxalic acid, 3 grams of Mn, 9 grams of P205 aswell as 2 grams of the potassium salt of the dibutyl ester of thedithiophosphoric acid. A light gray layer is developed.

Example 7 A sheet of refined steel containing 18% of Cr and 18% of Ni istreated for 3 minutes at a temperature of 80 C.

with a solution containing per liter 60 grams of crystal:

lized oxalic acid, 3 grams of Mn, 9 grams of P205 as well as 2 grams ofthe potassium salt of the dibutyl ester of thiophosphoric acid and 2grams of sodium bisulfite. A well formed, gray, uniform layer isdeveloped.

Example 8 A sheet of refined steel containing 18% of Cr and 8% of Ni istreated, for minutes, at a temperature of 90 C. to 95 C., with asolution containing per liter 60 grams of crystallized oxalic acid, 3grams of Mn, 9 grams of P205 and 2 grams of thiourea. talline layer isdeveloped.

Example 9 A sheet of refined steel containing 18% of Cr and 8% of Ni istreated, at a temperature of 60 C., with a solution containing per liter60 grams of crystallized oxalic acid, 3 grams of Mn, 9 grams of P and 2grams of thiourea as well as 3 grams of sodium bisulfite. After 3minutes a uniform crystalline layer is obtained.

Example 10 A sheet of refined steel containing 18% of Cr, 10% of Ni and2% of M0 is treated, for 10 minutes, at a temperature of 95 C., with asolution containing per liter 60 grams of crystallized oxalic acid, 3grams of Mn, 9 grams of P205 as Well as 2 grams of thiourea and 3 gramsof sodium bisulfite. The layer is uniformly crystalline Example 11 Asheet of refined steel containing 18% of Cr and 18% of Ni is treated,for 3-4 minutes, at a temperature of 60 C., with a solution containingper liter 60 grams of crystallized oxalic acid, 3 grams of Mn, 9 gramsof P205 as well as 2 grams of thiourea and 0.2 gram of the potassiumsalt of the dibutyl ester of the thiophosphoric acid and 3 grams ofsodium bisulfite. A uniform, dark layer is developed. When a temperatureof 90 C. is applied, the reaction is already finished after 1 minute.

Example 12 A sheet of refined steel containing 18% of Cr and 8% of Ni istreated, for 3-4 minutes, at a temperature of 90 C., with a solutioncontaining per liter 60 grams of crystallized oxalic acid, 3 grams ofMn, 9 grams of P205 as well as 2 grams of thioglycolic acid. A dark graycrystalline layer is obtained.

A thin, light gray crys-- 6 Examples 13-18 Additions to the bath perliter (vide table); chromiumnickel-steel containing 18% of Cr and 8% ofNi.

Examples oxalic Thiourea, sodium pH 1 acid, g. g. bisulfite, g.

NoTEs.-After about 5 minutes good development of the layer. Temperature:C. to 0.

Examples I9-31 Additions to the bath per liter (vide table);chromiumnickel-steel containing 18% of Cr and 8% of Ni.

Example oxalic P205, g. Thlourea,

acid, g.

sodium bisulfite, g.

t-nt-u- 9999995 95 999? NMMMMNNNNNNNN r vi w r-revi wer-OOOOOODOMOOOQDOOOWWMM r rr-z r-trr'r'm z- OQQOOQOOJCOQWQ No'rEs.Afterabout 5 minutes good development of the layer; temperature: 90 0. to 950..

Examples 32-35 Additions to the bath per liter (vide table); chromiumnickel-steel containing 18% of Cr and 8% of Ni.

Example oxalic P205, Fe thiourea, sodium pH acid, g. g. g g bisulfite,g.

No'rEs.-After about 5 minutes good development of the layer;temperature: 90 0. to 95 0.

Examples 36-38 Additions to the bath per liter (vide table);chromiumnickel-steel containing 18% of Cr and 8% of Ni.

Example oxalic, P205, Fe' thiourea, sodium pH acid, g. g. g g.bisulfite, g.

No'rEs.After about 5 minutes good development of the layer; temperature:90 0. to 95 0.

Examples 39-46 Additions to the bath per liter (vide table);chromiumnickel-steel containing 18% of Cr and 8% of Ni.

oxalic P205, thiourea, sodium Example 8010., g. Mn, g g. bisulfite, pH

NOTES.AIlI8I' about 5 minutes good development of the layer;temperature: 90 0. to 95 0.

aseaias "I z ama es. .7751

, Additions to r ebath. p r iter. (videtabls).;chzemisanickel-steelcontaining 18% of and of oxalic BzQr, ,Mn, Eef' thio sodiumExample a oid, g. g, g, urea, bisuipH 200 6.6 2v 0.1a 0.2- 1.8 r 1 .0

perature: 90- O. to 95 0.

Ex mples. 52: 3

Addit on o. he-hath er i er iewabl chromiumel. ontain ng 8% o Cr. and.of N NOTESPAME bm S QJHQP 99 1.61164 91 3 9 3? 2 i -W 3. 5

Norms-After about 5' minutes good development of the layer;-te1nperature: 80C. to 85 Q We la m:

1. A bath liquor for treating metal; surfaces in order tg. preparesurface layers, said liquorconsisting essentiallyof an aqueous solutionof oxalic acid; anions in an amount ranging from about 0.5, gram to.about 140. grams per liter, nd. an. am n ra g. rom about- 0.Q2 -.5 gramto about 5 grams per liter of at least one soluble'cofnpoundselectedfrom the group consisting of xanthates, dithiophosphor c. ac d. e e s othe, lip a ic. ser s. di hiophq phoric ac e r of he roma ic. ie t iq lyql c a id. and thiourea, said aqueous solution having an aciditycorresponding to a pH value in the range from about 1.0 to about 2 .4.

2. A bath liquor as claimedin claim 1,.wl1erein said soluble compoundisa xanthate.

3. A bath liquor; as cla med in claim 1, wherein said olubl compound isan organi este f. dimiq ho ph ric acid. a i

A bath q as cla medh 'Q ai l .whs' .ei a d soluble compound is hio lyeamacid.

A bath q or claimed n. laim 1, wherein. s id soluble compound isthiourea.

A bath q r asclia ued'ia cl im. .1 said liquo containing P04 ions.

7. A bath liquor as claimed in claim 1, said liquor containing ferricions 1 8-. A ba h quor a cl med. in c aim I, sai liq oreo nin ions, of.m n anese;

A ba h. i uo as c aimed. in c a m. 1. sa d. iq o ont in ng, as n a ceerato a l a t subs an e e as e rom the group. con ist Qt so iumsulii e.iam. bi ii e. an alkal meta thib u at n. alkalime al hydrosulfite, andan alkali metal thiocyanate;

10. The process for the treatment of metal surfaces in order to preparesurface layers which comprises contacting-the metal surface at atemperature between about 20- C. and about 98* C. with a liquorconsisting essen? tially of an aqueous solution of oxalic acid anions inan amount ranging from about 0.5 gram to about 140. grams per liter, andan amount rangingfrom about 0.025 gram to about 5 grams perliterof'atleast one soluble com.- pound selected from the group, consisting ofxanthates, dithiophosphoric acid esters o the aliphatic series,dithiophosphorie acid esters ofthe aromatic series, thioglycolic acid,and thiourea, said aqueous solution having an acidity corresponding toapH value in the range from about- 1.0; to about 2.4.

References Cited in. the file of this patent, UNITED STATES PATENTS

1. A BATH LIQUOR FOR TREATING METAL SURFACES IN ORDER TO PREPARE SURFACELAYERS, SAID LIQUOR CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION OFOXALIC ACID ANOINS IN AN AMOUNT RANGING FROM ABOUT 0.5 GRAM TO ABOUT 140GRAMS PER LITER, AND AN AMOUNT RANGING FROM ABOUT 0.025 GRAM TO ABOUT 5GRAMS PER LITER OF AT LEAST ONE SOLUBLE COMPOUND SELECTED FROM THE GROUPCONSISTING OF XANTHATES, DITHIOPHOSPHORIC ACID ESTERS OF THE ALIPHATICSERIES, DITHIOPHOSPHORIC ACID ESTERS OF THE AROMATIC SERIES,THIOGLYCOLIC ACID, AND THIOUREA, SAID AQUEOUS SOLUTION HAVING AN ACIDITYCORRESPONDING TO A PH VALUE IN THE RANGE FROM ABOUT 1.0 TO ABOUT 2.4.